Frequency modulation receiver squelch circuit



April 20, 1943; D. E. FOSTER 2 3 6,85

FREQUENCY MODULATION RECEIVER SQUELCH CIRCUIT Filed June 19, 1941 ATTORNEY Patented Apr. 20, 1943 UNITED STATES PATENT FFICE FREQUENCY MODULATION RECEIVER SQUELCH CIRCUIT Dudley E. Foster, South Orange,

to Radio tion of Delaware N. J assignor Corporation of America, a corpora- Application June 19, 1941, Serial No. 398,703

Claims. (Cl. 25020) relates to receivers of to wit: the suppression, or squelching, of interchannel noises.

Another important object of my invention is to provide an amplitude limiter for a frequency modulated carrier wave (FM) receiver which has the auxiliary function of preventing reproduction of electrical impulses in response to a decrease of received signals below a predetermined intensity value.

Another object of my invention is to apply noise squelching action to a point in the FM receiver system prior to the detector, and the squelching function being performed by a prior signal transmission tube.

Still other objects of my invention are to improve generally the simplicity and efliciency of noise suppression devices for FM receivers, and

economically manufactured and assembled in PM receivers.

into effect.

In the drawing:

Fig. 1 shows the circuit diagram of an embodiment of the invention,

Fig. 2 shows the limiter characteristic.

now, to the accompanying drawing, the invention is shown applied to theintermediate frequency (I. F'.) network of a super- 'a resistor connected deviated in accordance with the amplitude of the modulating signal, the rate of deviation beto a lower value of frequency; that is, the center frequency is reduced to the I. F. value. The latter may be of the order of 4.3 me. The frequencies given herein are purely illustrative and are not to be considered as limiting.

The FM waves, with the center frequency at I. F., may be amplified by amplifiers.

modulation signal applied to transmitter. the carrier Since the modulation appear on as a frequency variation of amplitude variation in the latter distortion. Hence, there is used a device, which has a flattened output above a, predetermined carrier input intensity, for eliminating the amplitude variation of the carrier. In Fig. 1 the limiter tube 6 is of the pentagrid type. Its cathode 2 is connected through a resistor R1 to a highly negative point on the direct current voltage supply potentiometer of the receiver. It is to be understood that R2 represents between the negative terminal of the direct current source and ground. The grid 6, located between the positive screen grids 3 and 5, is connected to the high potential side of the resonant input circuit Ill. The latter is tuned to the center frequency, the I. F. value, of the FM waves. The cathode 2 is connected to the low potential side of input circuit I The resonant input circuit It, which may be in the plate circuit of the converter or an I. F. amplifier tube, is tuned to the I. F. value. Circuits It and were reactively coupled so as to pass a band up to substantially 200 kc. thereby permitting the maximum frequency deviations to be transmitted to the detector.

The plate 6 is connected to the high potential side of circuit I2 which is tuned to the I. F.

value, and the low potential side of the circuit i2 is connected to the positive terminal of a direct current source. The grid I is connected to a voltage point which maintains the grid at a relatively low positive voltage. sayabout +2 Grids 3 and 5 are maintained at a positive potential of about +15 volts. The leads to each of grids I. 3 and 5 are by-passed for I. F. currents. The voltage supply lead to the plate may be by-passed to the cathode for I. F. currents. The output circuit I 2 is reactively couthe carrier at the the nal grid voltage applied the particular receiver' pled to the tuned circuit Hi. The latter is resonated to the I. F. value, and the coupling between circuits l2 and i3 is chosen to permit the network I2-l3 to pass the desired frequency deviations up to about 200 kc.

, The I.'F. amplifier tube 20 has its cathode grounded, while its signal control grid is connected to the high potential side of input circuit 13. A direct current blocking condenser is connected between the grounded cathode and the low potential side of circuit l3. The plate circuit of amplifier 20 may be coupled to the discriminator of any well known form of FM detector circuit. It is not believed necessary to describe the details of such a detector circuit, since those skilled in the art various types of FM detectors that may be uti lized. It is sufficient to point out that the FM detector circuittransforms the constant amplitude-variable frequency wave into a modulated carrier wave of constant carrier frequency and variable amplitude. The resulting amplitude modulated carrier wave is rectified to produce the modulation voltage, which may then be amplifled and reproduced.

The limiter characteristic is shown in Fig. 2. In that figure Plate current is plotted against Signal grid-volts. The symbol Ip designates the variation of plate current of tube I as the sigto grid 4 is varied in a negative direction and a positive direction relative to zero bias. It will be observed that beyond a predetermined signal grid voltage the plate current flowing in the output circuit of tube l substantially flattens ou It is sufficient for the purposes of this application to explain that the limiter circuit shown herein is operated so that the plate current is sharply cut off by negative signal grid voltage, and reaches saturation with a small positive signal grid voltage. Both polarities of the signal voltage are thus clipped in the tube, and the fundamental component of the I plate current is selected in the plate circuit.

Where the tube l is of .the 6A8 type, substantially constant output is secured above a predetermined signal grid voltage. The potentials of grids I and 3 control. the plate current at'which saturation occurs. grid 2' having a negligible effect because it consists only of supporting rods.

The potential of grid 3, which is internally connected to grid 5, is preferably between 10 and volts positive. course, be adjusted to meet the requirements of designated, it being pointed out that as grid l' is made increasingly positive the plate current saturates at higher values of grid 4 voltage and higher plate current. For maximum limiter sensitivity and flatness the signal should be clipped symmetrically on both polarities, and in the 6A8 tube the signal is clipped almost symmetrically without any bias on the signal grid. It vviIL therefore, be seen that the limiter tube is one of the space charge grid type and that plate current flows through'the resistor R1 when the limiter has signal voltage of significant amplitude applied thereto.

To provide noise suppression, when signal amplitude is below a predetermined tolerable level,

the low potential side of circuit I3 is connected by direct current voltage lead 30 to the cathode end of resistor R1. Therefore, the signal grid of tube is connected to a point of high negative voltage on resistor R2 through a path which includes the coil of circuit l3, lead 30 and resistor R1. Hence, when. no FM waves are bein are fully acquainted with the The limiter output may, of

received the current flow through limiter tube I is obviously substantially zero. Hence, there is a small voltage drop through resistor R1. Accordingly, it will be seen that in the no-signal state the bias applied to the signal grid of tube 20 is the high negative voltage across R: minus the small voltage drop across resistor R1. The negative voltage across section R2 is chosen so that it is sufficiently high to bias tube 20 substantially to plate current cut-01f. This prevents the transmission of any electric impulses to the following FM detector, Hence, interchannel noises, as well asnoises existingduring abnormally weak signal reception, are prevented from being reproduced. Further, tube. hiss will not be passed on to the detector.

However, when the FM waves applied to the limiter input circuit are of an amplitude above a predetermined level the plate current fiowing through R1 is a maximum, and, therefore, the voltage drop across resistor R1 is a maximum. The magnitude of resistor R1 is chosen so that at the amplitude of applied signals sufilcient to operate the limiter the drop across R1 reaches full value. In this state, the difference in voltage drop across R1 and R2 is such as to give normal bias to tube 20. That is, with limiting value of the plate current flow the voltage across resistor R1 will be sufliciently high in magnitude,

relative to the negative and opposite in polarity, voltage across R2 so that the bias on the signal grid of amplifier 20 will be reduced to the normal operating value for the amplifier.

In other words, when FM waves of proper strength are applied to the limiter the amplifier tube 20 is normally rendered effective or operative, and reproduction is had. It will, therefore, be seen that if at the limiting value or the plate current of tube 1 the effective bias on the signal grid of'tube ifl is normal, then with no signal applied to the receiver the effective bias on tube 20 will be cut-off bias with consequent low gain thereby decreasing the reproduction of noise and spurious responses. The present arrangement has the advantage over the application of other known forms of noise suppression circuits in that it is free from distortion; a better time constant can be designed in this circuit; and it will be quieter in operation.

While I have indicated and described a systerm for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims. What I claim isi i. In a receiver of angular velocity-modulated carrier waves of the type provided with at least first network, and rendering said space charge ,tube bias in response to normally preventing transmission through said transmission stage in the absence of frequency modulated carrier waves at the input of said limiter stage above a predetermined intensity level, deriving a control voltage from the limited current output of said limiter stage, and rendering transmission through said transmission stage effective in response to said control volt- 3. In a receiver of frequency modulated carrier waves, a, limiter network including an electron discharge tubeprovided with input and output circuits, a resistive impedance in the space current path of said limiter'tube, a transmission tube having an input circuit coupled to said limiter output circuit, means normally biasing said transmission tub to space current cut-off in the absence of frequency modulated carrier waves at said limiter tube inputcircuit above a predetermined intensity level, and means responsive to voltage of a predetermined magnitude developed across said resistive impedance for removing said cut-oil bias on said transmission tube.

4. In a receiver of frequency modulated carrier waves, a limite'r network including an electron discharge tube provided with input and output circuits, a resistive impedance in the space current pathof said limiter tube, a transmission tube having an input circuit coupled to said limiter output circuit, means normally biasing said transmission tube to space current cut-ofl in the absence of frequency modulated carrier waves at said limiter tube input circuit above a predetermined intensity level, and means responsive to voltage of a predetermined magnitude developed across said resistive impedance for removing said cut-oif bias on said transmission tube, said limiter tube being of the type including a grid at a small positive voltage, and having a small plate current'fiow in the absence of input signal voltage.

In a receiver of frequency modulated carrier waves, a limiter network including an electron discharge tube provided with input and output circuits, a resistive impedance .in the space current path of said limiter tube, a transmission tube provided with input and output electrodes and having an input circuit coupled to said limiter output circuit, means normally biasing said transmission tube to space current cut-ofl' iii the absence of frequency modulated carrier waves at said limiter tube input circuit above a predetermined intensity level, and means responsive to voltage of a predetermined magnitude developed across said resistive impedance for removing said cut-off bias on said transmission tube, and

said resistive impedance being included in -a common direct current voltage path between the input electrodes said cut-oi! bias means. -l

6. In a receiver of angular velocity-modulated carrier waves of the type providedwith at least two cascaded modulated carrier wave transmission tubes prior to the detection network, the method which includes (normally biasing the sec- 0nd of said transmission tubes'tospace current cut-off for modulated carrier waves of less than a predetermined amplitude, limiting the current output of the first of the transmission tubes to remove amplitude 'modulation effects, deriving a control voltage from the limited current output of said first tube, and reducing said second said controlvoltage.

of said transmission tube and I I waves, a limiter network direct current voltage having an ing 7. In a receiver of frequency modulated carrier including an electron discharge tube provided with input and output cirtransrmission tube provided with input and output electrodes and having an input circuit to said limiter output biasing said transmission tube to space current cut-oil in the absence tude developed across said resistive impedance for remoivng said cut-oil bias on said transmission tube, said limiter tube being of a type including a space charge grid at a small positive voltage and having a small space current flow in the absence of input signal voltage, and said resistive impedance being included in a common path between the input electrodes of said transmission tube and said cut-ofi bias means.

8. In a receiver of frequency modulated carrier waves, a limiter network including an electron discharge tube provided with input and output c ircuits, a resistive impedance in the space current path of said limiter tube, a transmission tu-be input circuit coupled to said limiter output circuit, means normally biasing said transmission tube to space current cut-oil in the absence of frequency modulated carrier waves at said limiter tube input circuit above a predetermined intensity level, means responsive to voltage of a predetermined magnitude developed across said resistive impedance for removing said cut-oil bias on said transmission tube, said biasing means and resistive impedance being arranged in polarity opposition.

mission tube to space current cut-01f in the absence of frequency modulated carrier waves at.

said'limiter tube input circuit above a predetermined intensity level, means responsive to voltage of a predetermined magnitude developed across said resistive impedance for removing said cut-oil bias on said transmission tube, said biasmeans and resistive impedance being arranged in polarity opposition, said limiter tube being of the, type including a space charge grid at a small positive voltage and having a small plate current flow in the absence 01' input signal voltage, and said resistive iinpedanc lbeing included in a common direct current voltage path between the input electrodes of said transmission tube and said cut-ofi bias means.

10. In a receiver of angular velocity-modulated carrier waves of the type provided with at least two cascaded modulated carrier wave transmission networks, the method which includes limit- 

